Field of the Invention
The present invention relates to an automotive door beam formed of an aluminum alloy extrusion with a closed-section structure.
Discussion of the Background
A door beam formed of an aluminum alloy extrusion typically includes a pair of flanges (outer flange located on the outside in a vehicle width direction and inner flange located on the inside in the direction) and a pair of webs connecting the flanges together. The door beam has a closed-section structure formed by the pair of flanges and the pair of webs (see Japanese Unexamined Patent Application Publication No. 2006-23336).
The aluminum alloy extrusion with such a closed-section structure is manufactured using a hollow die such as a porthole die, a bridge die, and a spider die. For example, an extrusion method using the porthole die uses a porthole die including a combination of a mandrel body having a plurality of portholes and a die. An aluminum billet is squeezed into the porthole die and divided into pieces by the portholes, and then the pieces are integrated again by welding while surrounding the mandrel. The inner surface of such a product is shaped by the mandrel while the outer surface thereof is shaped by the die, resulting in formation of an extrusion with a closed-section structure. Thus, welded portions necessarily exist in the aluminum alloy extrusion with the closed-section structure manufactured using the hollow die.
In the aluminum alloy extrusion having the welded portions, each welded portion has a microstructure different from that of any other portion (normal portion), and the welded portion disadvantageously has low mechanical properties, for example, a low fracture limit, compared with the normal portion. For a door beam formed of the aluminum alloy extrusion, such a low fracture limit of the welded portion may cause a reduction in strength and energy absorption of the door beam as an energy absorbing member.
The aluminum alloy extrusion with the closed-section structure having the welded portions has been investigated to improve mechanical properties of the welded portions by adjusting a material composition or a manufacturing condition as described in Japanese Unexamined Patent Application Publication Nos. Hei10(1998)-306338, 2003-154407, 2007-231408, and 2009-45672, for example.